Many designs of table lamps use a decorative molded hollow glass lamp base to support the electric bulb fixture. The glass base, typically formed by a press and blow molding operation, may for example be in the shape of a vase, a ginger jar, a figure, or another molded object. The socket fixture (which may also include the harp for mounting a shade) is secured at the top of the glass base. Usually the glass base itself rests on a cast metal base. The glass base is captured between the socket fixture at the top and the metal base at the bottom, by an axial tube which extends through the hollow interior of the glass base. The electric cord runs from the socket downwardly through the tube to the metal base and out through a side opening or tunnel in the metal base.
In many designs it would be desirable to lead the electric cord out from the glass base, rather than from a metal base. A side opening could be drilled in the glass, but drilling is a relatively slow and tricky operation in glass. For this reason the cord is usually carried out through a side opening or tunnel in a metal base, which is provided for that purpose beneath the glass base.
It is further inconvenient to secure the metal base directly to the glass base; usually the glass base must be sandwiched or clamped between the metal base and the bulb socket above it, by the central tube. The glass bases may be as much as 10-15" tall, or even taller, and the provision of so long a tube down the axis of the lamp is a significant added expense. Indeed, the provision of the metal base itself is a substantial expense.
It is known to provide an opening at the top of the glass base for the center tube by molding a knock-out plug in the glass. After molding and annealing the base, the inner end of the plug is knocked out, thereby forming an opening through the top of the base. The knock-out plug is formed by the use of a mold having an axially movable top hub section which can be withdrawn from the base after molding.